1. Field of the Invention
This invention relates to a thermal transfer printer for printing characters based on selectively heating exothermic elements of a thermal head so that ink of an ink ribbon is partially melted and transferred to a sheet of paper or other media (printing sheet) and, more particularly, to a thermal transfer printer in which an ink ribbon housed in a ribbon cassette is made to travel with stability, as well as to a ribbon cassette which enables the ink ribbon to travel with stability.
2. Description of the Related Art
Ordinarily, a thermal transfer printer has a construction in which a carriage is disposed on the front side of a platen so as to be reciprocatively movable along the platen, and a thermal head, capable of contacting the platen, and a ribbon cassette, housing an ink ribbon, are mounted on the carriage, the ink ribbon being lead out of an opening in the ribbon cassette so that an exposure section is positioned between the thermal head and a printing sheet placed between the ink ribbon and the platen.
In this type of thermal transfer printer, the ribbon cassette has a pair of rotatable bobbins around which the ink ribbons are wound from its extreme ends, and one of these bobbins serves as a draw-out bobbin from which the ribbon is drawn out of the ribbon cassette, while the other serves as a windup bobbin for winding up a portion of the ink ribbon used for printing.
In conventional printers, the wind-up bobbin is connected through a friction member, such as a felt member, to a rotary member which is driven by a motor mounted on the carriage or is moved based on the traveling of the carriage.
In this arrangement, during printing, the ink pinched between the thermal head and printing sheet is unwound from the draw-out bobbin and is successively used for printing as the carriage travels, and the portion of the ink ribbon separated from the printing sheet after being used for printing is wound up around the windup bobbin which is friction-connected to the rotary member.
However, in the case where the ink ribbon, separated from the printing sheet, is wound up by only the windup bobbin friction-connected to the rotary member as described above, the force of winding up the ink ribbon is changed with the change in the outside diameter of the portion the ink ribbon wound around the windup bobbin.
That is, as the outside diameter of the portion of the ink ribbon wound around the windup bobbin increases, the torque also increases so that the ink ribbon winding force becomes greater. It is therefore impossible to make the ink ribbon winding up force uniform. When the winding force is small, there is a risk of failure to separate the ink ribbon from the printing sheet with stability, or a risk of the ink ribbon being entangled, resulting in failure to perform printing of good quality with stability.